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Navy Air Cushion Vehicles (ACVs) Lift Fan Impeller Optimization


The Ship-to-Shore Connector (SSC), a replacement hovercraft for the existing fleet of Landing Craft, Air Cushion (LCAC) vehicles, utilizes a lift fan system to discharge air into the craft’s skirt and bow thrusters to lift the hovercraft under normal operation. Each SSC utilizes two identical lift fans which are defined by an impeller and a volute assembly. Each lift fan impeller includes a center disk, blades which are attached to both sides of the center disk, and two outer shrouds. Lift fan impeller blades are removable and replaceable without requiring the disassembly of the lift fan system. There is no commercial hovercraft of this scale and payload capacity. The current SSC lift fans meet craft performance requirements, but the SSC Program Office seeks the development of an advanced lift fan that increases fan efficiency by at least 10% while achieving minimal noise levels (125 db or less). Improvements in fan efficiency will increase fuel economy. SSC lift fan performance will be enhanced when impeller blade characteristics are optimized through their entire length. Current impeller blade characteristics, such as shape, and structure, affect aerodynamic losses and efficiency of the impeller. However, studies suggest (Ref. 1) conventional design methods, such as the streamline curvature of the fan blades, do not adequately address aerodynamic improvement. In addition, an extrusion manufacturing process used to form impeller blades also inhibits the development of an optimum shape. Each impeller blade includes a large attachment foot on each end of the blade which disturbs flow and reduces efficiency and mass airflow. Each inner lift fan impeller blade foot is attached to the fan center disk through a series of bolted fasteners and each outer lift fan impeller blade foot is attached to one of the two outer shrouds through a series of bolted fasteners. Although, undergoing dimensional modifications and other changes, the foot-to-blade transition area continues to negatively impact the aerodynamics within the lift fan system. This foot-to-blade transition promotes flow separation along the blade, and may change the angle in which the airflow is directed into the bow thruster and skirt supply ducts, which affects the efficiency of the fan by approximately 3-4% (Ref. 1). Extending the blades radially outward and away from the fastened area or a design which removes the footing while maintaining structural integrity are a couple of ways the lift fan system could achieve greater efficiency. Sweeping the blade shape may also improve efficiency (Ref. 2), however, areas to consider that may be affected by any changes to blade design are the blade leading and trailing edges, maximal flow, and direction of flow. There are little commercial alternatives to a Navy-grade centrifugal fan applied to an air-cushion system. Due to this limited availability, all performance aspects will be relative to legacy craft data and projected SSC estimates. The overarching goals of this effort are the development of a cost-effective advanced lift fan blade design and manufacturing process that will increase lift fan efficiency by at least 10% to optimize SSC fuel efficiency and reduce noise; extend SSC mission range; and minimize SSC production and life cycle costs. In addition, the Navy requires the removal and replacement of fan blades without the removal of the impeller from the SSC. The advanced lift fan impeller must meet the Navy’s fan specification (Ref. 3). PHASE I: The company will define and develop a concept for a lift fan impeller that meets the requirements as stated in the Description section above. The company will demonstrate the feasibility of the concept through aerodynamic modeling and analysis and show that the concept will provide a cost-effective lift fan for the Navy with improved fan efficiency and fuel economy. The company must also demonstrate the manufacturability of the fan. The concept must provide the capability for impeller blade removal and replacement without removing the impeller from its installed location on the craft. PHASE II: Based on the results of Phase I effort and the Phase II Statement of Work (SOW), the company will develop a prototype lift fan impeller for evaluation. The prototype will be evaluated to determine its capability in meeting the performance goals defined in the Phase II SOW and the Navy fan specification for efficiency, noise, and vibration. System performance will be demonstrated through installation and testing on SSC and by modeling and analysis. The fan must demonstrate increased fan efficiency (by at least 10%) and reduced noise level (below 125 db). The prototype will also need to be evaluated to ensure individual lift fan impeller blades can be removed without removing the impeller from its installed location on the craft. Evaluation results will be used to refine the prototype into an initial design that will meet the SSC Craft Specifications. PHASE III: The company will be expected to support the Navy in transitioning the lift fan impeller to Navy use on the SSC. The company will finalize design and fabricate production prototype lift fan impeller, according to the Phase III SOW, for evaluation to determine its effectiveness in an operationally relevant environment. The company will support the Navy for test and validation in accordance with SSC Craft Specifications to certify and qualify the system for Navy use and for transition into operational SSCs. Following testing and validation, the end design is expected to produce results outperforming the current SSC lift fan in regards to fan efficiency, air flow, and noise reduction.
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